alysia

I'm assuming 3 1/2 is a mixed fraction.

If you get confused with mixed fractions, you can always convert them to normal fractions like $\frac{1}{2}$. 

$3\frac{1}{2}$ converts to $\frac{7}{2}$. You can find this by multiply the denominator(bottom of the fraction) by the integer part then adding the numerator(top of the fraction). You get $3 \cdot 2 + 1 = 7$. Then divide that by the denominator or $2$ to get $\frac{7}{2}$. $\frac{7}{2} \cdot 20 = 70$ because the $2$ in the denominator cancels out with the $2$ in the $20$. Remember that $20 = 2 \cdot 10$ which is why they cancel. 

$70$ is your answer.